1. Field of the Invention
The present invention relates to a noise removal method for detecting and removing noise appearing in an image signal picked up with an image pickup sensor such as a digital camera.
2. Description of the Background Art
FIG. 10 is a schematic block diagram of a general digital still camera 100. In the digital still camera 100, an analog image signal picked up with an image pickup sensor 105 such as a CCD (charge coupled device) sensor or a CMOS sensor is converted to a digital signal and thereafter subjected to various image processing such as pixel interpolation, color space conversion and edge enhancement in an image processing part 106, as shown in FIG. 10. The Image data subjected to such image processing is culled and finder-displayed on a liquid crystal monitor 109 or the like, compression-coded in the image processing part 106 using JPEG (joint photographic experts group) compression-coding or the like and stored in a memory card 110 such as a nonvolatile memory, or output to an external device such as a personal computer through an interface 111. Referring to FIG. 10, numeral 101 denotes an optical lens, numeral 102 denotes a color correction filter, numeral 103 denotes an optical LPF (low-pass filter), numeral 104 denotes a color filter array, and numeral 107 denotes a driving part driving/controlling the image sensor 105 and the like.
However, a noise signal is disadvantageously mixed into the image signal picked up with the aforementioned image pickup sensor 105, to deteriorate the picture quality. When the image signal mixed with the noise signal is subjected to edge enhancement, the noise signal is also enhanced to further deteriorate the picture quality. This type of noise signal results from the hardware structure of an A/D converter or employment of pixel values around a noted specific pixel in pixel interpolation, for example. It is difficult to individually remove this type of noise signal, and hence the overall image signal is generally passed through the LPF 103 for blurring the noise signal appearing in the image signal. In this case, however, the edge-enhanced part is also blurred to disadvantageously reduce the overall picture quality.
According to a first aspect of the present invention, a method of removing noise from an image signal comprises steps of (a) detecting such a zigzag signal that the difference between the output levels of two adjacent pixels alternately takes positive and negative values at least three times in pixel arrangement along a prescribed direction and (b) noting a specific pixel among a plurality of pixels corresponding to the zigzag signal and calculating a mean value of the output levels of signals corresponding to the plurality of pixels with reference to the specific pixel for correcting the output level of a signal corresponding to the specific pixel to said mean value.
The method according to the first aspect can extract a noise signal having a zigzag output level appearing around a Nyquist frequency and individually correct the same. Therefore, the zigzag noise signal can be removed without deteriorating the overall picture quality.
According to a second aspect of the present invention, the zigzag signal is generated as a straight line in the image signal in the step (a), and the method calculates a weighted mean of the output level of the zigzag signal with reference to the specific pixel for correcting the output level of the signal corresponding to the specific pixel to the weighted mean in the step (b).
The method according to the second aspect can remove a generated noise signal as a straight line appearing around the Nyquist frequency from the image signal in higher accuracy.
According to a third aspect of the present invention, the image signal is a two-dimensional image signal formed by horizontal and vertical lines, and the zigzag signal is intersectionally generated in two directions of the horizontal and vertical lines in the image signal in the step (a), while the method calculates a weighted mean of the output level of the zigzag signal generated along the two directions with reference to the specific pixel at the intersection of the horizontal and vertical lines for correcting the output level of the signal corresponding to the specific pixel to the weighted mean in the step (b).
The method according to the third aspect can detect a noise signal at the intersection of two zigzag noise signals appearing in two directions of vertical and horizontal lines from a two-dimensional image signal and remove the same in high accuracy.
Accordingly, an object of the present invention is to provide a noise removal method for individually removing a noise signal mixed into an image signal without lowering picture quality.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.